The present invention relates to a flip chip package, a circuit board thereof and a packaging method thereof, and specifically to a flip chip package having an electrode structure suitable for a flip chip package, a circuit board thereof and a packaging method thereof.
An electronic circuit has been recently higher in density, and the need for a smaller area and connecting resistance has been intensified regarding a packaged device. One of means for attaining a high-density package is flip chip package. Several kinds of packaging methods are available for the flip chip package, and in consideration of easy repair and unleading, which has attracted attention in recent years, SBB method (Stud Bump Bonding) is a desirable technique. The SBB method is a technique for forming protruding electrodes, which are made of materials such as Au, on a semiconductor element by a wire bonding method and for connecting the protruding electrodes and electrodes on a circuit board via conductive resin.
Referring to FIG. 3, a paste electrode material is firstly printed on a circuit board 11 by a method such as screen printing, and the circuit board 11 is baked at a temperature of sintering the electrode material. Thus, circuit electrodes 12 are formed on the circuit board 11. On the other hand, protruding electrodes 14 are formed on a semiconductor element 13 by a method such as wire bonding, and a layer of conductive resin 15 is formed on the protruding electrodes 14 by transferring and so on. Thereafter, the circuit board 11 and the semiconductor element 13 are positioned high accurately and a suitable load is applied thereon. Hence, the semiconductor element 13 is packaged on the circuit board 11.
However, the above conventional packaging method is disadvantageous as in the following. First, since the semiconductor element 13 has decreased in electrode pitch in recent years, an electrode pitch on the circuit board 11 needs to be smaller accordingly. However, according to the conventional screen printing method, a pitch is limited up to 300 xcexcm and printing is difficult with a pitch below the limit, causing frequent short circuits and breaks in a wire. Consequently, the yields are lowered.
Secondly, when the semiconductor element 13 is smaller in electrode pitch, it is quite difficult to control a quantity of the conductive resin 15 transferred onto the protruding electrodes 14 of the semiconductor element 13. Particularly, short circuits are more likely to occur. In order to prevent the short circuits, a quantity of the conductive resin 15 is set smaller than that of the conventional art. However, since the electrode material expands laterally, the circuit electrodes 12 are each half-round in cross section when an electrode pitch is 100 xcexcm. In the case where flip-chip packaging is carried out on the circuit electrodes 12, the conductive resin 15 is squeezed out of the circuit electrodes 12 as shown in FIG. 3. Therefore, the conductive resin 15, which is squeezed out of the adjacent circuit electrodes 12, may be short-circuited.
The object of the present invention is to make a stable connection with a circuit board even when a semiconductor element has a small electrode pitch.
In order to solve the foregoing problem, when protruding electrodes formed on the semiconductor element are connected to circuit electrodes on the circuit board via conductive resin, the present invention comprises the steps:
(a) forming an electrode material film having a prescribed dry film thickness by using a paste electrode material containing a photopolymerizable material;
(b) exposing and developing the electrode material film;
(c) baking the developed electrode material film; and
(d) flip-chip packaging the semiconductor element on the circuit electrodes formed in the above steps.
In the above steps, concave circuit electrodes having warped edges are formed on the circuit board. Since the concave surfaces of the circuit electrodes act as saucers, it is possible to flip-chip package the semiconductor element without squeezing out any conductive resin. Consequently, it is possible to eliminate the occurrence of short circuits, achieving a reliable packaging for the semiconductor element.
Ceramic is applicable as the circuit board. The circuit board, can be provided not only with a circuit including circuit electrodes for mounting the above semiconductor element, but also a circuit for mounting components other than the semiconductor element as well as a circuit for making connection with another board.
Such an electrode material is used that contains, at least, metallic materials such as Au, Ag, or Cu and glass as inorganic components, and a monomer and a polymer which also serve as polymeric materials and a photoinitiator as organic components. Ni or Au plating may be applied to the surface of the formed circuit electrodes.
Such a semiconductor element is used that is formed with the protruding electrodes made of metallic materials such as Au, Al, Cu, and solder. Any method including wire bonding and plating is applicable for forming the protruding electrodes.
The conductive resin is applicable as long as the resin contains conductive components such as Au, Ag, and Cu to connect the protruding electrodes on the semiconductor element and the circuit electrodes on the circuit board. Any type of resin is applicable regardless of whether it is thermosetting or thermoplastic.
In the above step (a), a film is formed by printing a paste electrode material on the circuit board. The electrode material needs to be printed only on a package region for packaging the semiconductor element. The other regions may be formed with a circuit pattern in advance according to the above mentioned conventional method. In addition, upon printing, which is not intended for forming a circuit pattern, printing may be performed over, the entire surface of the package region. Thus, a rough printing such as a conventional screen printing is sufficiently applicable. The electrode materials is dried at a suitable temperature to prevent the material from flowing after printing. However, a printing plate and printing conditions need be set so as to secure a prescribed film thickness, which is preferably 10 to 20 (xcexcm).
In step (b), a glass mask and so on are positioned on the circuit board, on which the electrode material is printed and dried. The glass mask is formed, such that only an electrode region having the semiconductor element transmits light. Ultraviolet rays having a wavelength of 320 to 370 nm are radiated at 300 to 500 mJ. Thereby, polymeric materials on an electrode region which transmits ultraviolet rays are started by a photoinitiator to react and polymerized. After some appropriate time, the board is entirely developed by means of a solution for dissolving a polymeric material which has not reacted. Thus, the film is removed from other parts than the electrode regions and remains on the electrode regions. At this time, by adjusting the film thickness, polymeric materials receiving less light are left to remain insufficiently polymerized toward the board. Upon development, erosion is started from non-electrode regions from which the film has been removed, further to wider area toward the board. As a result, the film has a trapezoidal cross section on the electrode region. When a film thickness is small, all the polymeric materials are polymerized and it is impossible to obtain a film having a trapezoidal cross section. Therefore, the above film thickness is demanded.
In step (c), the circuit board completed with exposure and development is baked at a temperature for sintering the electrode material, and the circuit electrodes are baked onto the circuit board. At this time, the electrode material film is caused to slightly shrink, so that both ends of the formed circuit electrode are slightly warped (such warped part is called an edge curl) and the cross section of the circuit electrode becomes arc-shaped. Plating of Ni, Au and so on may be applied in order to protect the electrode surfaces after baking.
In step (d), the semiconductor element having the protruding electrodes formed thereon is flip-chip packaged on the circuit board by using the conductive resin. At this time, the edge curl of the circuit electrode acts as a wall, thereby, preventing the conductive resin from being squeezed out.